Biomedical Engineering Reference
In-Depth Information
probe to measure dose and dose rate. It can serve as a portable or stationary mon-
itor. Different probes can be attached for different radiations (alpha, beta, gamma,
and neutrons) and for different purposes, such as general surveys, surface contam-
ination monitoring, or air monitoring. The basic control module, which contains
extensive software, identifies the attached probe and automatically adjusts for it.
Proportional-counter tubes may be either of a sealed or gas-flow type. As illus-
trated schematically in Fig. 10.8, the latter type of “windowless” counter is useful
for counting alpha and soft beta particles, because the sample is in direct contact
with the counter gas. Figure 10.9 displays such a system that monitors tritium ac-
tivity concentration in air. The unit on the left regulates the incoming blend of the
counter gas (methane or P-10, a mixture of 90% argon and 10% methane) with air,
and also houses the detector and associated electronics. The unit on the right an-
alyzes and displays the data. Single pulses of tritium beta particles (maximum en-
ergy 18.6 keV) are differentiated from other events by pulse-shape discrimination.
Pulse-height discrimination with proportional counters affords an easy means
for detecting one kind of radiation in the presence of another. For example, to count
a combined alpha-beta source with an arrangement like that in Fig. 10.8(a) or (b),
one sets the discriminator level so that only pulses above a certain size are reg-
Fig. 10.8
Diagram of (a) 2
π
and (b) 4
π
gas-flow proportional counters.
